A parking brake device of this kind is discussed in DE 10 2008 007 877 B3. There, a spring brake cylinder is controlled by a relay valve which, for its part, is controlled by a bistable safety valve. The bistability is achieved by pneumatic feedback, with a select-low valve allowing the lower of two pressures applied to the outlet or to the inlet of the safety valve through to a control inlet of the safety valve. The safety valve is a 3/2-way valve having two inlets and one outlet. The first inlet can be connected selectively to a pressure medium source or atmospheric pressure by a first solenoid valve. The second inlet can likewise be connected selectively to a pressure medium source or atmospheric pressure by a second solenoid valve. The safety valve connects its outlet to one of the two inlets, depending on its operating position. The bistability of the safety valve is thus ensured, even if the power fails.
German patent document DE 10 2005 058 799 A1 discusses a parking brake device in which the spring brake cylinder is controlled by a relay valve which, for its part, can be controlled by a 3/2-way safety valve. This safety valve selectively connects its outlet to a compressed air source or atmospheric pressure. The purpose of this parking brake device is to enable the driver to activate the parking brake by actuating the service brake if the electric power supply fails.
German patent document DE 10 2007 061 908 B4 refers to a parking brake device having a spring brake cylinder, which can be selectively connected to a pressure medium source or atmospheric pressure by a relay valve. A pneumatic control inlet of the relay valve can be selectively connected to the pressure medium source or atmospheric pressure by a monostable changeover valve. The pressure medium outlet of the relay valve is connected to the pneumatic control inlet of the relay valve by a further monostable changeover valve. This provides a controllable feedback circuit for the relay valve, said circuit holding the relay valve pneumatically in a stable position in each case. It is thereby possible to avoid bistable valves for controlling the relay valve.
German patent document DE 103 36 611 A1 refers to a brake system having an electronic control device which operates electrically actuated valves upon receiving an electric actuating signal that requires deactivation of a parking brake function. In this case, use is made of a bistable valve which, in a first operating state, feeds pressure medium to an actuator and, in a second operating state, discharges pressure medium from the actuator.
Nowadays, parking brakes of commercial vehicles, including trailers, are normally fitted with spring brake cylinders which, in the release position, supply a spring compression space with compressed air and thus keep the spring under stress, while, for the parking brake function, the spring compression space is vented, i.e. connected to atmospheric pressure, with the result that the brake cylinder produces a braking force under the action of the spring (cf. Bosch, Kraftfahrttechnisches Taschenbuch (Automotive Handbook), 22nd German Edition, Düsseldorf, 1995, page 648).
In general, both purely pneumatically operated parking brakes, which are operated by driver-actuated parking brake valves that are generally bistable, and electropneumatic systems having a bistable electromechanical valve controlled by an electromechanical bistable valve are known. Both valve positions for “parking brake” and “release” must be “stable” here, i.e. must remain in the respectively selected position without human intervention. This also applies to failure of an electric power supply for the valves.
An electric or electropneumatic parking brake must therefore have two stable positions that can be maintained or automatically assumed, even if the power fails, namely                1. this state must be maintained without electric energy when the parking brake is applied;        2. the pressure to the spring-loaded cylinder and to the parking brake inlet of a trailer control valve must be maintained or supplied during a journey, at least while supply pressure is available.        
An unwanted changeover between the two states must be prevented, even in the event of a fault. The only exception permitted in this respect is that the spring brake is activated if a pneumatic line is broken off. In this case, the port leading to the trailer control valve must also be vented automatically.
The changeover from the parking state to the driving state and the changeover from the driving state to the parking state is to be triggered under electric control. It is furthermore desirable that it should be possible to set any pressure between zero and the supply pressure in the driving state.